1. Field of the Invention
The present invention provides methods and instruments for focusing and imaging x-rays using grazing incidence optics and medical uses thereof.
2. Background of the Invention
X-ray radiation is used for many medical applications. For example, radiation is used to kill tumor cells that are difficult or impossible to treat with surgery. This xe2x80x9cradio-surgeryxe2x80x9d usually employs what are typically classified as gamma raysxe2x80x94photons with energy in excess of 1 MeV.
Gamma rays are used (instead of x-rays with E less than 100 keV) because to kill diseased cells requires a dose of radiation comparable to the dosage needed to kill healthy cells. Thus, total dose to a tumor must exceed the dose to surrounding tissue (xe2x80x9ctherapeutic ratioxe2x80x9d) if the therapy is to be effective, i.e., kill the tumor cells but not damage healthy tissue. Currently, a proper ratio is achieved by directing the beam at the tumor from multiple directions by scanning the beam or pointing multiple beams at the tumor. By this method, the point where the beams cross receives a higher dosage than the healthy tissue the beams must pass through on the way. One problem with this method is that the beam intensity drops rapidly as it passes through the healthy tissue because of Compton scattering by electrons in the tissue. By the time an orthovoltage x-ray beam reaches the center of the brain, for example, it has been reduced to about 20% the intensity it had at the skin surface. Thus, it would require irradiation from five independent directions just to bring the flux at the center of the brain back up to the (presumably non-lethal) level of the skin. Because irradiation from so many directions is required to achieve a good ratio of tumor to skin exposure, the technique is problematical.
However, because of relativistic effects, Compton scattering drops by a factor of about two as the energy of the beam rises to the mega-volt range. With gamma rays, almost half the beam intensity survives to the center of the body. This allows improved exposure ratios between healthy and diseased tissues to be achieved with reasonable geometries. Additionally, the buildup effects of the relativistic Compton electrons keep skin dosage even lower, a highly desirable cosmetic result. However, problems caused by gamma rays include side scatter, over shoot, and low intensity beams. Precision below a target size of about 5 mm is not possible. By using a beam in the 50 keV range, the beam is cleaner, sharper and overshoots the target area less.
There is a rising incidence of breast cancer in the US, making it the leading cause of death for women in the 40 to 50 age group. One out of nine women will develop breast cancer in her lifetime. Mammography is being promoted as a major line of defense against breast cancer. Through early detection, women may have a better chance of receiving effective treatment. However, there are problems with mammography, and significant improvements are needed. The problems arise from the goal of imaging the soft tissue of the breast. A tumor is not significantly different in composition or density from the fibrous and glandular tissues of the breast, although it is of higher density than the adipose tissue whose content increases with age. For this reason, mammography is of limited value in young women where the incidence of cancer is lower, and the ability to detect a tumor is lower. The tumor usually becomes visible by eventually displacing the lower density adipose tissue. By this time, unfortunately, the tumor has become of substantial size. Improved contrast for seeing small tumors and improved ability to recognize unhealthy tissue at an early stage of development are central goals.
In order to enhance the contrast of the x-ray image, mammograms must use very low energy x-rays, typically 18 keV. The fractional energy absorbed by a small feature (e.g., a tumor) rises as the energy drops, creating higher contrast. Additionally, as the energy of the photons drops, the percentage absorbed in accordance with the photoelectric effect rises, reducing the scattered radiation component on the film or other recording medium, and thus enhancing contrast. Grids are usually employed as well to further reduce the scattered component. The problem with shifting to low energy photons is a rapid increase in the overall absorption of radiation in the breast. In a typical mammogram, less than 1% of the incident radiation emerges. As a result, the intensity of the radiation source must be increased by a factor of 100, subjecting women to much higher dosage in mammograms than is encountered with most diagnostic x-rays.
X-ray optics also can be used for microscopy. Indeed, the first grazing incidence optics (developed in the late 1940""s and early 1950""s) were applied to microscopy. There is a variety of reasons why x-ray microscopy is of interest, but the field has been stalled for lack of a practical imaging system
The first rationale for x-ray microscopy is simply resolution. As Fraunhoffer proved early in the 19th Century, the resolution of a microscope is fundamentally limited at about one wavelength of radiation. Thus, light microscopes (those that use visible radiation) are limited to about 0.5 xcexcg resolution. A shift to the ultraviolet can extend resolution to about 0.2 xcexc. The very short wavelengths of x-rays can potentially break through this barrier.
A second relevant property of x-rays is their ability to penetrate matter. Thus, unlike electron microscopes, an x-ray microscope can be used on live cells in an air environment. In addition, x-ray absorption is dependent on material density and elemental composition. Thus, changes in absorption across K edges can lead to contrast enhancement, creating an exquisite sensitivity to elemental composition not found with light or electrons.
The design of an x-ray microscope, using grazing incidence optics, is analogous to the design of a conventional light microscope. A light source is placed below a slide containing a sample. The radiation penetrates the sample, and is partially absorbed. The light diverging from the sample is re-imaged and magnified by an optic. A detector is placed at the focal plane to record the image.
The present invention solves the problems discussed above, and others related to medical and microscopic uses of x-rays, by providing methods and instruments for medical uses of focused and imaged x-rays.
The present invention relates to medical and microscopic (together xe2x80x9cbiologicalxe2x80x9d) uses of focused x-rays. More specifically, the invention relates to use of focused x-rays for radio-surgery, mammography and microscopy. The x-rays may be focused using grazing incidence optics or other x-ray optical methods. A preferred method employs spherical mirrors for such grazing incidence optics. Even more specifically, the optical system disclosed in U.S. Pat. No. 5,604,782 may be employed. Also provided are apparatuses for use in the methods. In particular is provided a radio-surgery, a mammography and a microscopic instrument employing grazing incidence optics.